Poster: Secondary metabolism
631: The Carotenoid Pathway of the Primitive Red Alga Cyanidioschyzon merolae
Gantt, Elisabeth , Contact Author||Authors||Cunningham, Jr., Francis X. (A) Lee, Hansel (A) Gantt, Elisabeth (A) |
(A): Dept. Cell Biology & Molecular Genetics, University of MD, College Park, MD 20742|
The primitive unicellular red alga Cyanidioschyzon merolae 10D, a resident of acid hot springs, provides perhaps the best approximation of the photosynthetic apparatus of the ancestral procaryotic chloroplast progenitor. The pigment composition of C. merolae, ascertained by HPLC, was found to be relatively simple, with Chl a, zeaxanthin, and β-carotene as the major pigments, and β-cryptoxanthin as a minor constituent. Carotenoids with ε-rings (i.e. lutein and α-carotene), found in many of the more advanced red algae, were not detected in pigment extracts of C. merolae. The ε-ring cyclases of green plants are related to the β-ring cyclases and produce β-ringed carotenoids as minor products. The cyclase enzymes of certain cyanobacteria also yield a mixture of products with β- and ε-rings. We therefore examined the β-ring cyclase of C. merolae for evidence of any cryptic ε-cyclase activity. No ε-cyclase activity was observed: only β-ringed carotenoids were produced when lycopene was provided as the substrate for the C. merolae enzyme expressed in Escherichia coli. The nuclear genome of C. merolae [Matsuzaki et al., (2004) Nature 428, 653-657] contains plant-like carotenoid pathway genes for enzymes leading only as far as β-carotene. A homolog of plant genes that encode the enzyme that converts β-carotene into zeaxanthin (β-carotene hydroxylase) is not present. Instead, this enzyme in C. merolae may be specified by a chloroplast gene that encodes a polypeptide related to cyanobacterial-type β-carotene hydroxylases. An examination of the enzymatic activity of this putative C. merolae β-carotene hydroxylase is in progress.